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Author: Subject: PABA > Aniline
BobHawson
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[*] posted on 1-3-2009 at 15:08
PABA > Aniline


How would one one convert PABA (paraaminobenzoic acid) into aniline?



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[*] posted on 1-3-2009 at 17:26


Decarboxylating its salt with sodium hydroxide would probably work.

[Edited on 1-3-2009 by kclo4]




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Sauron
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[*] posted on 1-3-2009 at 17:30


Decarboxylate the PABA without oxydizing the readily oxidizable amino group, which would be a difficult trick.

As you were informed when you started a thread on acetaminophen -> aniline, you are driving against the traffic and on wrong side of the road.

Both are difficult transformations. Particularly in presence of a sensitive group.

Better to obtain or make benzoic acid, decarboxylate that to benzene, nitrate that to nitrobenzene and reduce that to aniline.

All of those are easy well documented transformations.

If you can acquire benzene or nitrobenzene directly do so. Hoppe's #8 bore solvent for cleaning the rifling of firearms is mostly nitrobenzene, you might try distilling that (in a fume hood or outside.)

Toluene boiled with potassium permanganate solution will get you benzoic acid.

Benzoic acid -> benzene has been subject of a thread, UTFSE.

Ordinary nitrating mixture (conc sulfuric and conc nitric acids in correct proportions and amounts) gets nitrobenzene, again outside or in a hood, nitrobenzene is harmful.

Tin and conc HCl reduces nitrobenzene to aniline, much more harmful than nitrobenzene, be warned!




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kclo4
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[*] posted on 1-3-2009 at 17:50


What Sauron said is the smartest thing to do, but perhaps you could oxidize the PABA to 4-nitrobenzoic acid and then decarboxylate that and then reduce it. I don't think the NO2 would survive though, but it seems like a possibility.



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[*] posted on 1-3-2009 at 18:23


My expectation would be a flask full or tar.

Protecting the NH2 with formyl, benzoyl or acetyl might help but somehow I doubt the thread author has formic acid, benzoyl chloride or Ac2O on hand.




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[*] posted on 1-3-2009 at 19:48


I'm curious, what would the tar contain, or how would the tar form?
Would the nitro group oxidize parts of the molecule, or form a salt similiar to how nitromethane and sodium hydroxide do?




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Sauron
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[*] posted on 1-3-2009 at 20:46


I am referring to the oxidation of PABA to p-nitrobenzoic acid. That sounds like a tar baby to me.

If you managed to get isolated 4-nitrobenzoic acid then there is no reason to expect further tar formation.

Reducing aryl nitro compounds to amines is facile, but oxidizing arylamines to nitro compounds is generally not a piece of cake. Last time I looked into this a rather costly catalyst was advised.

[Edited on 2-3-2009 by Sauron]




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BobHawson
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[*] posted on 1-3-2009 at 21:57


Could you please link to the thread covering benzoic acid into benzene? I was unable to find it.

And couldn't you get benzoic acid from sodium benzoate with HCl as well as from toluene and potassium permanganate?




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[*] posted on 1-3-2009 at 22:06


Benzene summary thread

https://sciencemadness.org/talk/viewthread.php?tid=4902

And rather than

benzoic acid => benzene => nitrobenzene => aniline

I would consider

benzoic acid => ammonium benzoate => benzamide => aniline

the conversion to amide being done by heating with B(OH)3 catalyst in xylene, the water formed being azeotroped off. Chlorine bleach will convert the amide to aniline.
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[*] posted on 1-3-2009 at 22:14


Why not just start with Sodium Benzoate?
http://www.sciencemadness.org/member_publications/benzene_pr...

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[*] posted on 1-3-2009 at 23:20


"Decarboxylate the PABA without oxydizing the readily oxidizable amino group, which would be a difficult trick."

im not doubting it, but i dont understand. like the C02H group oxidizes the amine to the Ph-N-N-Ph diazo hydrazine thing? and in turn goes to aldehyde?

i thought some copper salts eased decarbyoxylation, but i dont really understand the mechanism behind decarboxylation, ionic..radical..i dont know

an acidic solution in a very high boiling point solvent like glycerin and the protonated amine should be effectlivly inert, no?
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[*] posted on 2-3-2009 at 00:12
CORRECTION


Upthread I stated that a commercial product for cleaning the bores of firearms, Hoppe's #9, is primarily nitrobenzene.

This is erroneous.

Maybe it was true at one time, but I just looked up the MSDS for this product and it contains no nitrobenzene at all.

It is composed of kerosene, ethanol, amyl acetate, xylene, NH4OH, and other ingredients.

We used to refer to this as "Nitro" in the old days and it smelled of nitrobenzene strongly. I also used a great deal of surprus military bore cleaner, same smell. These were not kerosene and alcohol and banana oil etc.

Anyway I just wanted to go on record that harvesting nitrobenzene from this product is a nonstarter.

[Edited on 2-3-2009 by Sauron]




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[*] posted on 2-3-2009 at 00:20


In literature there are only examples using the Cu/quinoline method for decarboxylating p-aminobenzoic acids (for example: Synthesis (1985) 778-779; JACS, 129 (2007) 4824-4833).
You can't just heat a carboxylic acid and expect it to decarboxylate just like some magic thing. There must be a viable mechanism, either a SET mechanism as in Cu/quinoline method or one of the several other mechanistic possibilities, but all are substrate specific (none is general).

PS: I'm moving this thread to the Beginnings section due to the original poster not using references and obviously not using neither the search engine or literature.




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[*] posted on 2-3-2009 at 08:25


BobHawson,

One of the dead giveaways you have an old nitro bore cleaner, is the smell is very similar (yet peculiarly different) than benzaldehyde. Still, it is similar enough that if you have old bore cleaner, and it smells like bitter almonds, you probably have the old nitro formulation. If you look in the reagent aq section, you can see benzoic acid is readily available from cider mill supplies.
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[*] posted on 5-3-2009 at 00:12


PABA > p-nitrobenzoic acid clean and easy with acidic dichromate, very standard procedure - consult vogels.

No tar, 70% yield, did it in second year as the first step to semi-synthetic benzocaine.

[Edited on 5-3-2009 by Siddy]
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[*] posted on 5-3-2009 at 05:09


Maybe by Kolbe electrolysis...?

Aromatic carboacid decarboxylate in such conditions...while aliphatic do a kind of dimerisation...
2R-CO2H --> R-R + 2CO2 + H2
C6H5-CO2H --> C6H6 + CO2

...so maybe?
H2N-C6H4-CO2H --> C6H5-NH2 + CO2




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[*] posted on 11-6-2011 at 03:38


What is obtained by electrolysis of sodium p-aminobenzoate? Maybe amino group wouldn't survive, but sodium p-nitrobenzoate could be electrolysed to make nitrobenzene?
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[*] posted on 1-3-2021 at 22:08


Quote: Originally posted by Nicodem  
In literature there are only examples using the Cu/quinoline method for decarboxylating p-aminobenzoic acids (for example: Synthesis (1985) 778-779; JACS, 129 (2007) 4824-4833).
You can't just heat a carboxylic acid and expect it to decarboxylate just like some magic thing. There must be a viable mechanism, either a SET mechanism as in Cu/quinoline method or one of the several other mechanistic possibilities, but all are substrate specific (none is general).

PS: I'm moving this thread to the Beginnings section due to the original poster not using references and obviously not using neither the search engine or literature.


Does anyone else believe this or have doubts?
https://m.youtube.com/watch?v=S0T-sR-nn2Y
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[*] posted on 2-3-2021 at 10:48


So.... Perhaps it would be better to actually try an experiment, rather than just declare an endeavor impossible?

I always figured it was possible. I just didn't like the high price of PABA.

I like the experimental technique. Even though, I'm a little rusty, I think I could handle it.

Another positive... No shipping problems! Lord knows what hazard fees might be for shipping Aniline.

One of the supplement companies, is selling PABA at $47.00 US/kilo....

Oh, and thank you for your post.... It was illuminating.

Well enough, for some Gold to be out there.. Somewhere.

Very helpful, for someone to find it, and holler "Eureka!"

Swiped from Draculic.... https://m.youtube.com/watch?v=S0T-sR-nn2Y









[Edited on 3-3-2021 by zed]
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[*] posted on 2-3-2021 at 22:05


I always wondered if this would work but I always thought of
Na benzoate+NaOH = benzene so
PABA+NaOH = aniline
Thing is in the video no base is used so I was dubious.
But if it works awesome.
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[*] posted on 3-3-2021 at 17:21


Well, my concern would have been polymerization. That is what happens, when you heat Glycine neat.

Yes, Glycine does decarboxylate... Some. But, it also produces a flask containing plenty of brown crud. And, a distillate of semi-solid white material, appearing to be principally Methylamine Carbonate.

Perhaps PABA's aromatic amino-group is less subject to amidization. That demo looked like a pretty clean conversion.

The thermal decarboxylation of Niacin works to produce Pyridine, though it requires a catalyst to work well.
And, the Nitrogen present is less vulnerable to attack.

The procedure looked pretty legit to me. Why would this guy fake it?

I'd like to give it a test run. Just because I have wondered about it for so long. But, if It actually works as advertised... Well, then I'd have Aniline on my premises. And, I don't want Aniline on my premises. At least, I don't think I do.

Hmmm. Poisonous. Oily liquid. No earthly use for it. Oh, wait a second. I could use it to make Phenyl Hydrazine. Easy make, and it might be relatively difficult to buy now. I 've got a lingering interest in the Fischer Indole synthesis.

Always wondered what pure Skatole really smells like.

Hmmm. Maybe I'll try it. There is a nifty new method for producing Phenyl Hydrazines. Involves reducing a Benzene Diazonium salt, with ascorbic acid.

So... Easy Aniline.

Guess I better check on the availability of Phenyl Hydrazine.


[Edited on 4-3-2021 by zed]
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[*] posted on 4-3-2021 at 00:27


Im just amazed at how many ppl have thought of paba to aniline but no one has tried it until now.
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[*] posted on 4-3-2021 at 03:29


After asking if anyone else has done this or does he have any posts, papers, references etc of anyone actually doing this before ( lots of ppl have theorised this but no actual experiments) I got this reply where he says other ppl have replicated it and it has a the corresponding products made from it and has the same rxns as aniline so I'm satisfied it's genuine aniline formed.im convinced it's legit: here's the response:

One the reasons I wanted to film this was because I hadn't seen it done before and it's practically useful for the amateur chemist. I started looking into this reaction about two years ago after running the decarboxylation of niacin to pyridine, I figured that a similar thing could be done for paba to aniline. I was unable to find any papers using simple thermal decarboxylation of paba but I did find similar papers demonstrating the thermal decarb of anthranlic acid to aniline here: https://cdnsciencepub.com/doi/pdf/10.1139/v52-065 In early 2019 first tried decarboxylation in aqueous H2SO4 but I only got a tiny yield of what I presumed to be aniline from its immiscibility in water and subsequent solubility in acid. Moving onto thermal decarb, I then figured that I may as well try just heating the solid before experimenting with adding Cu chromite or anything else. To my surprise, I found that simple thermal decarb yielded an oily liquid with chemical and physical properties corresponding to aniline. I've used the aniline from this to make quinoline and phenylhydrazine which I then made into tetrahydrocarbazole in another video, also the product has a hypergolic reaction with nitric acid. My procedure has been replicated by a number of people on Explosions&Fire's discord server as well.

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[*] posted on 4-3-2021 at 10:58


Below is a link to the first page of a 1963 article describing the decarboxylation of molten PABA and other acids. Unfortunately I can not retrieve the full article at the moment.

https://pubs.acs.org/doi/abs/10.1021/j100795a033




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[*] posted on 4-3-2021 at 11:35


Quote: Originally posted by draculic acid69  
Im just amazed at how many ppl have thought of paba to aniline but no one has tried it until now.


The most common things are:

1) People speak about something, but their life carries on and they never return to it, or
2) they come asking, meanwhile gather data on the web, try it and make it work, they either need it for some purpose or something, and never return to tell here that they got it working, because they got what they wanted.

I find it annoying that people come to be spoon-fed for some synthesis, but never input any of their own info. I've slowly gained a decent info on all of the basic stuff and still going on, and every now and then I come up with these simple, otc synthesis for useful reagents and memorize them.

I suppose I'm gonna write up a bunch of synthesis during this year once I get everything else in my life order and make some time. I've already got a list ready.

-

But, as salicylic acid readily decarboxylates to phenol, and niacin turns into pyridine with help of a little catalyst, this would be a logical continuation.

[Edited on 4-3-2021 by Fyndium]
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